(a) Field of Invention
This invention relates to the preparation of non-degradable hydrogel membranes that form polymer networks swollen in water and are suitable for organ restoration or replacement. The invention also relates to hydrogel systems consisting of a crosslinked network of hydrophilic co-polymers swollen in water or in biological fluids, which are well tolerated by living tissues and can be used in a wide range of biomedical applications. The invention also relates to hydrogels which show dimensional integrity with a water content between 50 and 88%, and viscoelastic properties similar to biological tissues, and which, because of their ability to retain a substantial amount of water with respect to network density, allow the transport of small molecules and nutrients. In addition, the invention also relates to porous hydrogels having low interfacial tension with biological fluids, and structural stability which make them suitable for implantation in soft tissue and in contact with biological fluids such as the cerebrospinal fluid or the blood. The present invention also relates to the use of the above hydrogel membranes to replace or restore the dural membrane when surgical removal of part thereof is needed after traumatic, neoplastic, inflammatory destruction or to correct a congenital defect. It is also useful for wound closure of the palate cleft and regeneration of the defect which aid in the healing of the tissue palate, and for delivery of cells to the eye when part of the cornea or the retina need to be restored.
(b) Description of Prior Art
1—Dura Mater
Dura mater is a membrane, which is found between the skull and the brain. It is also present between the vertebral column and the spinal cord, where it ensures protection against leakage of the cerebrospinal fluid (CSF). Any defect of the dura mater can produce undesirable consequences such as brain herniation, adhesion formation between the neural tissue and the overlying structures, pseudomeningocele, cortical scarring, CSF fistulas and wound infection with potential propagation to the brain parenchyma. Dural defect often requires a dural substitute when there is insufficient dura, for example when a large defect is created in the dural envelope for example in the course of tumour removal. Also, congenital anomalies such as Arnold Chiari malformation and myelomeningoceles and spinal dysraphic states may require a duraplasty as part of the repair. Therefore there is a need to repair such defect with a membrane that can mimic the functionality characteristics of the dura mater and that meets surgical need requirements such as sterile and suturable conditions.
Methods have been developed as an attempt to achieve an efficient closure of dural defect that include various materials selected from the following categories: (i) autologous tissues and allografs and xenografts including viable and nonviable membranes comprising fascia lata, pericranium, temporalis fascia, allantoic membrane, amnioplastin, cartilage membrane, cat gut, lyophilized human cadaver; (ii) alloplastic materials that include metalllic materials comprising aluminium foil, gold foil, nickel plate, platinum foil, silver foil, stainless-steel plate, tantalum; (iii) resorbable materials comprising biological polymers such as collagen, alone or complexed with α-hydroxy acids or methacrylate polymers, elastin-fibrin materials, and synthetic copolymers derived from α-hydroxy acids (iv) non resorbable polymers such as aliphatic polyurethane and polytetrafluoroethylene and polysiloxane-carbonate block copolymer; (v) lattice work of knitted monofilament polypropylene mesh, polyester and silicon composites.
In category (i) according to U.S. Pat. No. 4,400,833, a dural patch is described which utilise heterogeneous animal tissue comprising tendon or ligaments from cow or other animal. Also, in J. Neurosurg 61, 351 (1984) there is disclosed the use of a porcine dermis as a dural substitute. The major drawback of such material is health hazard due to the risk that that animal tissue may vehicle viral agents or prions disease such as Creutzfeld-Jakob disease or bovine spongiform encephalopathy as previously reported. In addition, these materials may create adhesions as a result of severe inflammatory response, and therefore are not safe for human use. Dural substitutes of category (ii) have been used in the past century but are no more used because of their inadequacy and since polymeric materials are preferred with the advance in new biomaterial polymers for artificial organs.
In category (iii) U.S. Pat. No. 5,997,895, relates to a collagen matrix to be used as dural substitutes and in U.S. Pat. No. 5,861,034 a bioabsorbable artificial dura mater is described which is made of lactic acid and ε-caprolactone. Both Patents disclose a non-stable dural substitute that degrade in vivo, and this is characterised by a persistent acute inflammation at 2 weeks. In particular, collagen induces an acute inflammation and a foreign body reaction that leads to a granulation tissue. This inconvenience may necessitates a second operation for tissue removal, cleaning and restoration of the defect. Degradation of poly(lactic acid) proceeds by autocatalytic hydrolysis of unstable functional group, e.g., ester groups, that causes the release of low molecular weight oligomeric products, in contact with the neural parenchyma and bone skull, as well as in the CSF and in the systemic circulation. Ultimately their accumulation in various organs of the body may cause some form of organ failure (e.g. kidneys) over a long period of time. In addition, the degradation causes local nonbacterial inflammatory reactions with activation of macrophage and foreign body giant cells. As a result, the device loses the structural integrity which is associated with its primary function, and therefore its functional integrity. Finally, since various factors influence the rate of degradation of biodegradable polymers, such as pH, ionic strength and the pressure of the environment, and also the geometry and dimension of the device, the rate of degradation and the subsequent rate of production of debris products cannot be controlled after implantation, and consequently the performance of the device cannot be controlled.
However, most of studies are not extensive enough for an appraisal of long term evaluation of degradation of poly (a-hydroxy acids) since the phenomenon of foreign-body tumorigenesis has been observed in rodents after 24 months [Nakumara et al., 1994; Pistner et al., 1994]
In category (i) according to U.S. Pat. No. 4,400,833, a dural patch is described which utilise heterogeneous animal tissue comprising tendon or ligaments from cow or other animal. Also, in J. Neurosurg 61, 351 (1984) there is disclosed the use of a porcine dermis as a dural substitute. The major drawback of such material is health hazard due to the risk that animal tissue may vehicle viral agents or prions disease such as Creutzfeld-Jakob disease or bovine spongiform encephalopathy as previously reported. In addition, these materials may create adhesions as a result of severe inflammatory response, and therefore are not safe for human use. Dural substitutes of category (ii) have been used in the past century but are no more used because of their inadequacy and since polymeric materials are preferred with the advance in new biomaterial polymers for artificial organs.
Dural substitutes of category (iv) are formed of elastomeric materials that eventually may induce formation of neomembranes and are usually sterilized with ethylene oxide gas that can leave residual toxicity. In addition, a problem of watertight has been reported with elastomeric material for dural closure.
Therefore an entirely satisfactory dural substitute remains to be developed. In order to establish an efficient, reliable and safe method for dura augmentation and replacement, the substitute should be non-toxic, non-absorbable, non-resorbable, biologically and chemically inert it should not induce revitalisation of the implant by the surrounding tissues, it should be non adherent to the underlying neural tissue, non irritative, and resistant to ingress of infections. In addition, it should be readily sterilized preferably by autoclave as other currently used methods may lead to changes in properties (toxic residual ethylene oxide) and structures (radio-induced chemical changes), handled and suturable and achieve a watertight closure with the healthy dura mater. It should also be pliable and easy to cut to any specified dimensions and conform easily to the surface of the brain or spinal cord. It should have a high tensile stress or strength and be suturable. It should not support cell adherence, ingrowth and proliferation and remain independent from the neural tissue. It should also provide an effective barrier for the wound against exogenous micro-organisms. It should be manufactured as mass marketable.
2—Palate Cleft
Palate clefts are congenital malformations of the palate due to a failure of the lateral palatine processes to fuse with each other, with the nasal septum, and/or with the posterior margin of the median palatine process.
Surgical treatment of palate clefts (palatoplasty) is a major surgery, which necessitates the isolation of the mucoperioteal flaps of the lateral palate to close the defect. This leads to the formation of important scars, which subsequently impede the normal development of the superior maxillary. This, in turn, causes a pseudoprognathism, which necessitates a major orthodontic treatment. Therefore there is a need to develop an effective method to substitute the classical methods of palatoplasty surgery which aims at correcting the tissue defect of the palate without impeding the normal development of the superior maxillary, and at reducing or preventing scar formation with restoration of velopharyngeal function. The use of an hydrogel membrane to correct the palate cleft hold great promise as a means of creating prosthetic materials since, on the one hand, it allows the closure of the tissue defect and, on the other hand, it provides a polymeric template to guide mesenchymal cells of the tissue palate for in situ repair of the defect during the development of the craniofacial skeleton.
3—Substrate for Cell Delivery to the Eye
Description of Prior Art
The retina which comprises about 150 millions cells is a neural membrane which transmits light stimuli to the brain via several neuronal pathways and relays. The transduction process into which light stimuli are transformed in nerve impulses (action potentials) which are carried to the optical nerve involves several classes of cells organized in layers and that comprises layers of rods and cones, horizontal, bipolar and amacrine cells and a layer of ganglion cells. The retina proper includes three layers of neurones: (a) rods and cones, (b) bipolar cells, and (c) ganglion cells. Light reaches the stratum opticum first and after traversing all the other layers affects the rods and cones in the outermost layer. The nerve impulse which results from the stimulation of the photoreceptors of the retina then passes through the layers in the numerical order given above up to the ganglion cells which form the terminus of the optic nerve. The optical nerve transmits the signal to the lateral geneculate nucleus, which then transmits it to the occipital lobe of the brain. The central part of the retina is the macula lutea and at the centre of the macula there is the fovea centralis (about 1.75 mm2) where conditions for photopic vision are optimum and the highest visual resolution takes place.
Some cause of visual loss include age related macular degeneration (ARMD) which is associated with dysfunction of the retinal pigment epithelial (RPE). The RPE is a monolayer of tightly coupled epithelia cells at the outer layer of the retina. The RPE cells maintain the health of the photosensitive cells of the retina by controlling the flux of electrolytes and small molecules between the blood and the neural retina. Therefore, the disappearance or the dysfunction of RPE results in photosensitive cell death. To overcome this and to rescue photoreceptor cells, one treatment option for maintenance or restoration of retinal function, consists in transplantation of RPE cells as a monolayer beneath the retina to replace the defective cell monolayer. To achieve this, a substrate material is essential to ensure that the RPE cells are in correct polarity with respect to one another and to the photosensitive cells of the retina in order to maintain tissue-specific organisational features after transplantation. In addition, the substrate should be handled by the surgeon without damage to the cells and should have a size to fit the defective zone. Prior to transplantation, the RPE cells must be seeded onto the substrate materials. This procedure allows to verify the viability of cells.
Thus, various substrates have been used in an attempt to attach and hold together RPE cells and to deliver RPE cells to the retina. Anterior lens capsule [Hartman et al., 1999], collagen [Bhatt et al., 1994], gelatin [Huang et al., 1998], fibrinogen [Oganesian et al., 1999], Bruch's membrane [Tezel et al., 1999], biodegradable polymers such as poly-L-lactic acid and poly-lactic-co-glycolic acid) [Hadlock et al., 1999; Lu et al., 1998 and 2001] have been used as carriers to transplant RPE cell monolayers. However these substrates are not suitable in a biological and surgical perspective, principally because they are not biostable after transplantation and they degrade and resorbe over time, releasing low molecular weight oligomeric products, which in turn may cause a retinopathy. In addition this causes the migration of the cell graft away from the site of transplantation. These substrates are also difficult to handle by the surgeon without risk of damage of the carrier device. In addition, those being of biological origin may induce immune rejection after antigen exposure, which cause failure of the graft.
As an alternative to eliminate these problems, there is a nee for a biostable, non-degradable, semi-rigid hydrogel membrane amenable to manipulation during surgical implantation of crosslinked synthetic polymers, of about 30-60 microns thickness with cell adhesivity properties which can be used as a support and proliferation for RPE cells. Through a pars plana sclerotomy into the posterior portion of the eye, this hydrogel-cell hybrid implant could be inserted into the subretinal space via a small retinotomy. Neural retinal cells can also be seeded onto such hydrogel susbtrate for transplantation and restoration of retinal function. Ultimately, the use of such a hydrogel would provide a reliable substrate for co-seeding both RPE and retinal cells and then induction in vitro of a biohybrid retinal substitute for the remplacement of the part of the retina. The cells can be obtained either from the patient using a retinal flap in the para- or perifoveal region, from a donor (embryonic retinal cells) or from an established cell culture lines
Corneal endothelium cells cover the front of the cornea and maintain the cornea as a transparent refracting surface. Dysfunctional corneal endothelium results in corneal opacification and decreased vision. The treatment is the transplantation of the whole cornea (keratoplasty). However, an alternative approach would be to replace only the functional part of the cornea, i.e., the endothelial monolayer. This could be achieved by seeding corneal endothelial cells onto an artificial substrate for subsequent replacement of defective parts of the cornea. The cells can be obtained either from the patient, from a donor or from established cell culture lines.
Various approaches have been proposed to transplant corneal epithelium such as cell monolayer on a cross-linked gelatine membrane [McCulley et al. 1980]. But gelatine, which is composed of denatured collagen, may undergo biodegradation in vivo and resorption that may lead to complications and infection, as well as the lost of the graft.